Plate-like morphology is realised as a result of preferential growth of particles in crystallographic directions parallel to certain planes and slower growth mechanisms in directions other than this plane.
It is known that particles with a particular morphology can be generated by heat treatment in certain flux or diluent systems where the product phase is soluble in the flux or diluent. For example it has previously been shown that plate-like particles of bismuth tungstate can be grown during heat treatment of bismuth oxide and tungsten oxide in molten salt mixtures at temperatures above 650° C. Molten chlorides promoted plate-like growth, whereas molten sulphate salts did not. It is not known in the art to generate plate-like particles in solid state systems.
Plate-like particles have a significant potential for a wide range of applications including soft focus filler materials for cosmetics, polishing and lapping slurries, advanced ceramic materials, composites, hard coatings, paper coatings, die coatings, and substrate materials for pearlescent pigments.
Plate-like particles with a high aspect ratio and a low degree of aggregation and/or agglomeration are particularly attractive for these applications. “Aggregation” is a term that refers to the degree to which particles are loosely bound together to form clumps which have an increased effective particle size. “Agglomeration” is a term that refers to particles which are rigidly bound to each other, for example by partial fusion or intergrowth. As for aggregation, the effective particle size is increased. The degree of aggregation and/or agglomeration of particles is routinely assessed using a combination of scanning electron microscopy and laser light scattering particle size analysis. Close agreement between the average particle size determined by these two methods indicates that the particles being tested are substantially discrete ie have a low level or degree of aggregation and/or agglomeration.
Processes for the production of plate-like particles of alumina are known in the art. For example, platy alumina is commercially produced by the controlled calcination of aluminium trihydrate. A hydrothermal technique for the production of plate-shaped alumina particles is described in U.S. Pat. No. 5,587,010. With known techniques, the particles tend to be agglomerated, requiring the use of expensive post milling and classification processes (see for example U.S. Pat. No. 3,121,623 and U.S. Pat. No. 5,277,702) to separate the particles. The particles also tend to be of irregular shape and size, with minimum dimensions exceeding 1 micron and thus a relatively low aspect ratio.
Certain of the techniques directed specifically to achieving high aspect ratios and forming substantially discrete plate-like particles known in the prior art require uneconomical conditions of temperature and pressure to form plate-like particles. An example of such a technique is disclosed in U.S. Pat. No. 5,702,519 (Merck) which describes a process for the production of non-aggregated plates of alpha alumina with a high aspect ratio. The Merck patent discloses a process which involves the step of preparing aqueous solutions of water-soluble aluminium and titanium salts. Complete dissolution of these salts is essential to avoid agglomeration of the final product. The starting materials, ie the salts, are completely dissolved as a first step. The process of Merck is preferably conducted at high temperatures between 900 and 1400° C.
The present invention was developed with a view to providing an alternative process for the production of substantially discrete plate-like alpha alumina particles with a high aspect ratio.
It will be clearly understood that, although prior art use and publications are referred to herein, this reference does not constitute an admission that any of these form a part of the common general knowledge in the art, in Australia or in any other country.
Throughout this specification and in the appended claims, the term “comprising” is used inclusively, in the sense that there may be other features and/or steps included in the invention not expressly defined or comprehended in the features or steps subsequently defined or described. What such other features and/or steps may include will be apparent from the specification read as a whole.